1. Field of the Invention
The present invention relates to a gas turbine that is provided with an abradable honeycomb seal installed to reduce the leakage of working fluid from a clearance between turbine blades and a casing.
2. Description of the Related Art
In a gas turbine mainly including a compressor, a combustor, and a turbine, high-temperature combustion gas flows between turbine blades installed on a rotating shaft and stator vanes installed on a stationary casing side. It is desirable to prevent the combustion gas from leaking from the turbine blade tip clearance in terms of the performance of the turbine. To meet such request, a gas turbine provided with an abradable honeycomb seal secured to the casing side is generally used. This honeycomb seal is manufactured in the following manner: a thin sheet metal is processed to form a corrugated shape in which trapezoids are alternately continued; a plurality of the corrugated thin sheet metals are overlapped at their walls of node, the walls of node being brazed with each other; and the abradable honeycomb seal provided with a number of approximately hexagonal voids are manufactured.
FIG. 7 is a schematic view exemplifying a seal structure located at the tip of a gas turbine blade B using an abradable honeycomb seal H. The honeycomb seal H is secured to a shroud S on a casing C side at a portion facing a seal fin F installed on the tip of the rotating turbine blade B. A clearance between the tip of the seal fin F and the honeycomb seal H is maintained as little as possible. The leakage of combustion gas at the tip of the turbine blade B is accordingly suppressed.
As illustrated in FIG. 8, the whole of the honeycomb seal H is generally configured as follows: a corrugated thin sheet metal P is processed such that trapezoids are alternately continued; a plurality of the corrugated thin sheet metals P are overlapped at their walls of node K; and the walls of node K are brazed with each other (at the brazed places R) so that the corrugated thin sheet metals P are secured to each other.
The honeycomb seal H is formed of a material relatively softer than the turbine blade B. The honeycomb seal H comes into contact with the rotating turbine blade B when the rotating turbine blade B extends in the radial direction perpendicular to the rotational axis due to thermal expansion. As a consequence, the honeycomb seal H is easily abraded by the seal fin F arranged on the tip of the turbine blade B. Accordingly, while avoiding the damage and vibration of the turbine blade B, the clearance between the turbine blade B and the honeycomb seal H is kept constant whereby the leakage of combustion gas is suppressed.
Incidentally, as regards the securing of the honeycomb seal H to the shroud S on the casing C side, as illustrated in FIG. 8, the honeycomb seal H is generally secured to the shroud S via the brazed place R in such a manner that the longer direction of the wall of node K (also the longer direction of the brazed place R extending in the Y1 direction in FIG. 8) may coincide with the rotational direction (the Z direction in FIG. 8) of the turbine blade B. The reason of this securing method is as follows: the corrugated thin sheet metals P formed with press working or other methods are secured to the shroud S so as to extend in the rotational direction of the turbine blade B. Hence, the securing-workability of the corrugated thin sheet metals will be satisfactory and manufacturing efficiency will be enhanced.
However, in the securing configuration of the honeycomb seal H described above, the walls of node K and the brazed places R which are abraded when the seal fin F abrades the honeycomb seal H become larger in length. The abradability of the honeycomb seal H by the seal fin F will be degraded accordingly.
When the rotating seal fin F comes into a situation of contact with the brazed place R, the abradability of the honeycomb seal H by the seal fin F significantly lowers compared with the case of contact with only the corrugated thin sheet metal. In fact, the thickness of the seal fin F is equal to or greater than that obtained by addition of the thickness of the brazed place R to the thickness of the two walls of node K. The seal fin F therefore will simultaneously abrade the two walls of node K and the brazed place R between the walls along the longer direction.
As illustrated in FIG. 8, the brazed place R of the honeycomb seal H is linear because the wall of node K is one side of a trapezoid. The longer direction of the brazed place R and the sliding direction of the seal fin F along with the rotational direction of the turbine blade B (and the seal fin F) are almost the same direction. Thus, a distance becomes long in which the seal fin F comes into contact with and slides along the brazed place R with low abradability. In addition, the abradability of the honeycomb seal H by the seal fin F lowers because of the formation in which the longer direction of the brazed place R independently faces the slide of the seal fin F.
The seal fin F may be abraded by the brazed place R in some cases. With the seal fin F being abraded, the leakage of the combustion gas from the abraded portion increases, which directly leads to a decrease in the performance of the gas turbine.